Identification of novel intracellular and microenvironmental signals that influence adult neurogenesis is a major research priority. Here we provide evidence for a novel role of cyclin dependent kinase 5 (Cdk5) in adult neurogenesis. This is striking since, unlike other cyclins, Cdk5 and its cofactors p35 and p39 are enriched in post-mitotic neurons. Based on our preliminary data, we hypothesize that Cdk5 activity in the adult hippocampus regulates proliferation and maturation of adult neural precursors. To test this hypothesis, we propose three aims to be accomplished in two years. Aim 1: Define protein expression of Cdk5 and required cofactors in subgranular zone (SGZ) precursors as they mature. We have validated the use of a novel Cdk5 antibody for protein analysis, including immunohistochemistry. Guided by our preliminary data, we will define when Cdk5 protein and its required cofactors, p35 and p39, are expressed in maturing SGZ cells. Aim 2: Determine the impact of Cdk5 ablation from precursors and their progeny on neurogenesis. New pilot data with our nestin-CreERT2 mouse suggest that inducible loss of Cdk5 from SGZ precursors and their progeny is detrimental to discrete stages of adult neurogenesis. We will determine how transgenic-mediated, precursor-specific ablation of Cdk5 impacts the a) number, b) phenotype, and c) morphology of these cells as they mature into dentate gyrus neurons. We hypothesize that loss of Cdk5 from precursors and their progeny will negatively impact their differentiation but have no effect on proliferation. Aim 3: Explore the impact of Cdk5 ablation from dentate gyrus neurons on SGZ neurogenesis. We show new data that loss of Cdk5 in mature dentate gyrus neurons has profound negative effects on SGZ precursors and immature neurons. We will test the hypothesis that Cdk5 in mature neurons in the hippocampus is critical for maintaining the SGZ neurogenic niche. We will use viral-induced ablation of Cdk5 to quantify the impact of Cdk5 ablation from mature dentate gyrus neurons on the a) number and b) morphology of SGZ precursors and immature neurons. These studies will provide fundamental insight into the potential cell autonomous effects of Cdk5 on adult neural stem cell development (Aims 1, 2), and the cell non-autonomous effects of Cdk5 in mature neurons on adult hippocampal neurogenesis via alteration of the neurogenic microenvironment (Aim 3). These studies will shed much-needed light on the physiological and pathological functions of Cdk5, and potentially indicate a highly novel role for Cdk5 in regulation of adult hippocampal structure and function. Contrary to a long-held belief that the brain cannot regenerate, the adult brain is now known to give rise to new neurons throughout life. Understanding the cues that influence these adult-generated neurons is critical, as it may hold the key to understanding brain functions, like learning and memory, and may allow us to harness these new neurons for repair of the injured or addicted brain. Here we propose to explore the impact of a novel regulator, cyclin dependent kinase 5, on the birth of new neurons in the adult brain.